How 1 plus 1 equal 3

I went along on family camping trip with my family and joining the family was my brother's family along with his sister-in-law. She was a year younger than and fairly quiet, the weeklong trip we were having fun and drinking a bit as well. On the fourth night we really partied, and I awoke in my tent with my brother's sister-in-law. Both naked and sticky and I was a little hung over, but I realized we had fucked at least once. Anyway, too late to cry over spill sperm. As a result, I'm marrying my sister-in-law sister next Saturday before it becomes too apparent, she is pregnant. OH well at least she lets me fuck her regularly.